Currently, optical fibers used for optical fiber communication systems which are generally spreading adopt a structure in which an outer periphery of one core is surrounded by a clad, and information is transmitted when an optical signal propagates in this core. Further, as the optical fiber communication systems spread, the amount of information to be transmitted is dramatically increasing in recent years. Following an increase in the amount of information to be transmitted, the optical fiber communication systems use several tens or several hundreds of multiple optical fibers to perform long-distance optical communication of a large volume.
It is known that, to reduce the number of optical fibers in such an optical fiber communication system, a plurality of signals are transmitted by means of light propagating in respective cores using a multicore fiber in which outer peripheries of a plurality of cores are surrounded by one clad.
Following Non-Patent Document 1 discloses an example of such a multicore fiber. In this multicore fiber, a plurality of cores are arranged in one clad. However, as is pointed out in Non-Patent Document 1, in the multicore fiber, optical signals propagating in respective cores interfere each other, and noise is superimposed on the optical signals propagating in the respective cores in some case. Hence, Non-Patent Document 1 discloses changing relative refractive index differences of the respective cores with respect to the clad as one method for reducing crosstalk between the cores.